This page documents the process to update ESXi from version 3.5 to 4.0 if you have a standalone host or do not use vCenter Update Manager. After you have completed the firmware upgrade, you should also upgrade the virtual hardware version of your VMs as shown at the bottom of the page and also upgrade VMware Tools. Upgrading the virtual hardware version will allow you to take advantage of new VM features like 10 virtual NICs per VM, IDE virtual hard drives and VMDirectPath.

This procedure to upgrade an ESXi host from version 3.5 to 4.0 requires version 4.0 of the VMware vSphere client. Typically the vSphere client is installed after your ESXi host is installed. if you don't have an existing install of the vSphere client v 4.0 you can extract it from the ESXi 4.0 upgrade ZIP package (VMware-viclient.vib\data.tar.gz\data.tar\.\4.0.0\client\VMware-viclient.exe) as shown in the below image.

When installing the vSphere client, ensure that you choose the option to install the vSphere Host Update Utility 4.0.

After the install, start the vSphere Host Update Utility application. When start the application, you will be prompted to download patches from VMware. You can manually check for patches using File \ Download Patches from VMware.

If you ESXi 3.5 host is not shown, you can select Host \ Add.

Select your host and then click on upgrade. The upgrade wizard will start and you may have to select an upgrade ZIP package as shown below. Click on next after the package has been selected and the package will then be validated. After the upgrade package is validated you'll be able to accept the ESXi EULA, followed by a prompt for the root login to the ESXi host. Note that all VMs on the host should be stopped and the host should be in maintenance mode. It's also a very good idea to take a host configuration backup with vicfg-configbackup.pl from the RCLI.

If all your VMs are shutdown and the host is in maintenance mode, the a host compatability check will run and the you can then click Finish on the Ready to Complete screen to start the upgrade.

As the upgrade proceeds the Host Update Utility will display the progress. During the upgrade process the host will reboot and you may experience a long delay in the upgrade process at about 34% done. It may appear that the upgrade has hung, but it will still be running.

Once the upgrade is complete the host will be running ESXi 4.0. A reboot will not be required, but you will have to take the host our of maintenance mode before you can start using your VMs again. It may also be necessary to update your license configuration if for example you were using a central license host and had not upgraded that prior to the host upgrade. You can also optionally upgrade the virtual hardware of your VMs before you power them on again but it's best to do that after upgrading VMware Tools in the VM. Note that if the virtual hardware upgrade for your VM is taking a long time, it's a good idea to check the Summary page for the VM for any messages as shown in the last image below.